Generally, many cellular and other communications systems operate in a full duplex manner—simultaneous transmission and reception—using different frequency bands to transmit and receive, known as Frequency Division Duplex (FDD). As illustrated below, there is a frequency separation between transmit and receive bands. In a wireless transceiver, information is transmitted at a power level that is typically many times higher than the received power. Interfering energy, which is generated by transmitter nonlinearities and other noise processes, is leaked into the receive band frequencies where it interferes with desired reception. This interference is referred to as self-interference in this description. Proper receiver operation requires attenuation of this self-interference, in many cases on the order of 100 dB or more.
Conventional mechanisms for attenuation of self-interference energy in the receive band of FDD systems include filter-based duplexers (air dielectric or ceramic filters) and diplexers, which are based on the ability to attenuate some frequencies while passing other frequencies due to the frequency separation between transmit and receive bands. For example, FDD wireless base stations require a duplex filter to separate transmit and receive signals to/from a single antenna. Duplexers allow simultaneous transmit and receive operation through one antenna due to frequency-dependent filters between transmit and receive ports. One drawback is these components are typically quite large to meet the isolation requirements of modern communications systems. Isolation requirements often exceed 100 dB between the transmit power amplifier (PA) and the receive Low-Noise Amplifier (LNA) that share the same antenna. This requires the use of air-cavity filters roughly 10″×10″×2″ and larger. Multi-band operation of such systems requires complex and limited multiplexer designs or a very large switch network of duplexers. These duplexers are typically constructed of a metal housing with air-filled internal cavities and are therefore large and heavy. Multiple operating bands are also desired. This further exacerbates the linearity problem due to the difficulties in matching the PA for best linearity vs. power over a wide bandwidth.
The continued worldwide adoption of 4G adds other challenges, both in terms of the proliferation of bands and the increased interference emitted from the PA. This increased interference requires larger duplexers. Further, 4G operates over wider bands, which increases the challenge of self-interference cancellation schemes. In addition, present art multi-band transceivers require the use of multiple duplexers which in the present art are switched in and out for operation at a given band of operation of the base station. The trend for cellular waveforms is an increase in the requirement for transmit-to-receive isolation, which requires more difficult filter requirements. Another trend is the increase in number of defined bands. For example, LTE has nearly 40 defined bands. This can require a large number of filters for a multi-band transceiver. These problems are exacerbated when the transceiver operates at high powers, for example, micro-class and macro-class base stations.
More recently, attempts have been made to remove self-interference in non-FDD full duplex systems (where simultaneous transmission and reception occurs in overlapping frequencies) by using electronic means or a combination of passive and active electronics means. Recent work has focused on a solution for the cases where transmit and receive operations occur in the same frequency band. Heretofore, Time Domain Duplexing (TDD) architectures were employed in such cases, separating transmission and reception in time, rather than frequency. The present schemes provide an electronic means for attenuating the transmit signal at the receiver input. These schemes employ a balun-based or transformer-based coupler to provide to the receiver canceller a sample of the interference that is 180° out-of-phase and free from additional group delay. These schemes assume that the phase of the transmit signal seen by the receiver is constant across the band. In the full duplex FDD case, the phase of the transmit signal seen within the receive band is not constant and is a function of frequency, known as group delay −∂Φ/∂ω. Accordingly, beyond the edges (or skirts) of the transmit signal, the energy which falls within the receive band is non-linear and the phase of the transmit signal shifts across the receive band. However, these full duplex, non-FDD systems cannot operate for existing defined cellular standards or other wireless communication standards that separate transmit and receive energy based on frequency (FDD) or time (TDD).
In radio frequency transmission systems which employ modulation schemes in which symbols contain both in-phase and quadrature-phase components, distortion exists between in-phase, I, and quadrature-phase, Q, components of a symbol constellation in the transmission between transmitter and receiver. In order to optimize the performance of the system in terms of improving error rate performance of the transmission channel this distortion is typically compensated for. Uncompensated for distortion with respect to both phase and/or amplitude results in a received constellation in which the constellation is ether rotated in phase and/or offset in amplitude between I and Q components.
What is further needed is software defined radio front end which can be reconfigured to operate in different bands and/or in different modes of operation with reconfiguration under software control.